Method of producing coated abrasive particles

ABSTRACT

A discrete coated abrasive particle wherein the coating comprises an inner metallic, e.g. nickel, layer and an outer nonmetallic protective layer which is preferably a solid resin such as phenolformaldehyde resin is produced by depositing the protective layer on to metal coated particles and the metal coated particles are preferably washed with a cleaning solution immediately prior to the deposition.

United States Patent Roy et al. 1 1 Dec. 2, 1975 METHOD OF PRODUCINGCOATED 3,528,788 9/1970 Seal 51/295 ABRASIVE PARTCLES 3,585,013 6/1971Bruschek 51/295 3,645,706 2/1972 Bovenkerk et al.. 51/295 1 1 Inventors:Alexander Rose y, 16 Cecil 3,650,714 3/1972 Farkas 51/295 Meredale,Johannesburg, Transvaal; 3,650,715 3/1972 Brushek et al.. 51/295Johannes Daniel Van Schalkwyk, 3,661,544 5/1972 Whitaker 51/295 904Total Centre, Braamfontein, Johannesburg, Transvaal, both of South Af iPrimary ExaminerDonald .1. Arnold Attorney, Agent, or Firm,Young &Thompson [22] Filed: Jan. 14, 1974 [21] Appl. No.: 433,242

[57] ABSTRACT [30] Foreign Application Priority Data June 5, 1973' SouthAfrica 73/3773 A discrete coated abrasive particle wherein the coat-Jan. 22, 1973 South Africa 73/0450 ing comprises an inner metallic, e.g.nickel, layer and an outer non-metailic protective layer which isprefer- [52] US. Cl 51/295; 51/298 ably a solid resin such asphenolformaldehyde resin is 1 324D /0 B24D 3/28 produced by depositingthe protective layer on to [58] Field Of Search 51/295, 298 metal coatedparticles and the metal coated particles are preferably washed with acleaning solution imme- [56] References Cited diately prior to thedeposition.

UNITED STATES PATENTS 3,518,068 6/1970 Gillis 51/295 8 Claims, NoDrawings PARTICLES 1 r This invention relates to the coating of abrasiveparticles.

Resin bond abrasive wheels consist essentially of -a metal hub portionaround the periphery of which is provided a grinding portion. Thegrinding portion consists of a plurality of abrasive particles held "orembedded in a resin matrix. .The abrasive particles arein general eitherdiamond or cubic boron nitride and the resin is generallyeither:phenolformaldehyde resin or a poly-, imide resin. 1

Such resin bond abrasive wheels are used for the wet and dry grinding ofsteel, carbide or like workpieces.

It is well known in the art that the retention properties of theabrasive particles in the resin matrix can be significantly improved bycoating the particles with a suitable metal, In the case of wet grindingapplications, nickel is the preferred metal, whereas in the case of drygrinding copper is the preferred metal.

The wheel manufacturer does not, in general, coat the abrasiveparticles, but purchases the metal coated particles from another sourcefor incorporation into wheels. There may well be a significant period,eg many months, from the time the particles are coated to the time thecoated particles are incorporated in resin bond wheels. During thisperiod, the exposed surface of the metal coating tends to oxidise andthese oxides, it has been found, impair the bond formed between themetal and the resin.

According to this invention, there is provided a method for producing adiscrete coated abrasive particle in which the coating comprises aninner metal layer and an outer non-metallic protective layer. Theprotective layer will be substantially inert to normal environmentalconditions and will bond readily to the resin matrix of resin bondwheels. Preferably the protective layer is a solid resin. Suitableresins are phenolformaldehyde, epoxy, polyimide, alkyd, non-saturatedpolyester and polyamide resins.

With this protective layer it has been found the coated particles can bestored for many months without significant deterioration; such particlesalso bond better to the resin matrix of wheels.

The metal of the inner layer will be any metal suitable in the field ofmetal clad abrasive particles. As mentioned above, the preferredmaterial is nickel in the case of wet grinding applications and copperin the case of dry grinding applications.

The metal layer will generally be present in an amount of from 20 to 80percent by weight of the coated particle. The protective layer willgenerally be present in an amount of from 1 to 3 percent by weight ofthe coated particle this provides the particle with a thin continuousprotective layer.

According to a more particular aspect of the invention, there isprovided a method of producing coated abrasive particles in which thecoating comprises an inner metallic layer and an outer non-metallicprotective layer, which method includes the steps of providing a batchof metal coated particles, depositing the protective layer on the metalcoated particles and recovering the particles as discrete particles eachhaving a metal/protective layer coating.

Preferably, the metal coated particles are treated prior to theprotective layer deposition substantially to remove any metal oxidesfrom the exposed metal surface. The deposition should occur as soonafter-the treatment as possible so' as to'av'oid, as far as possible,any reoxidation of the metal. This treatment will, in general, alsoremove other impurities such as greases and results in improved bondingbetween the metal layer and the protective layer.

The treatment is preferably a washing of the particles with a cleaningsolution, preferably an alkaline cleaning solution. 'A suitable cleaningsolution is a 10 to 30 percent by weight sodium carbonate cleaningsolution. It has been found that elevated temperatures improve thecleaning powers of the solutions. Thus, it is preferable that thecleaning solutions are at or about their boiling points for thetreatment;

The invention is illustrated by the following examples.

EXAMPLE 1 Na Si 0;,

H O 24.33% NajCO 65.01% Impurities Balance The nickel coated grit wasrefluxed for half an hour and then removed and washed with distilledwater. The particles were dried with analytical reagent acetone andimmediately thereafter placed in a solution consisting of 40%phenolformaldehyde dissolved in 60% tetrahydrofurane, percentages beingby weight. The particles and solution were agitated by stirring and theresin allowed to deposit on the metal coated particles and set. Theamount of solution was so chosen that the total quantity of resin forthe 35 carats of diamond amounted to 0.24 grams. With this amount ofresin, the particles each had a thin continuous layer of resin amountingto about 3 percent by weight of the coated particle. The particles wererecovered as discrete coated particles.

The coated grit as prepared above was incorporated into resin bondabrasive wheels in the usual manner. The wheels were compared withwheels prepared using standard /100 US. mesh nickel coated grit of theprior art. The results of these comparative tests are tabulated below:

As is well known, the G-ratio is a measure of the retentive propertiesof the diamonds in the resin matrix. The higher the G-ratio the greaterthe retentive properties. It is clear from the above results thattreating the 3. nickel coated diamond grit in the manner described inExample 1 provides unexpectedly good results, even in cases where thecoated grit is stored for a period of two months.

EXAMPLE 2 ,35 carats of 80/100 U.S. mesh nickel coated grit (nickelcoating 55 percent by weight of the coated particle) was washed withImperial Zenith Cleaner in the same'manner as described in Example 1.Immediately after washing the coated grit was placed in a saturatedsolution of a commercially available polyimide resin intetrahydrofurane. The mixture was stirred and the resin allowed todeposit on the particles and set. The mixture was filtered and theparticles recovered as discrete, nickel/polyimide resin coatedparticles.

We claim:

1. A method of producing coated abrasive particles for use in themanufacture of a resin-bonded abrasive tool in which the coatingcomprises an inner metallic layer and an outer solid resin protectivelayer, which method comprises the steps of providing a batch ofparticles coated with a metal selected from the group consisting ofnickel and copper present in an amount from to 80% by weight of thecoated particle, removing metal oxides from the exposed surfaces of theparticles,

immediately thereafter depositing on the particles, before reoxidationof the exposed surfaces thereof occurs, a protective layer of a solidresin selected from 4 the group consisting of phenolformaldehyde, epoxy,polyimide, alkyd, non-saturated polyester and polyamide in an amountfrom 1 to 3% by weight of the coated particle, and recovering theparticles as discrete particles having a metal/protective layer coating.

2. A method according to claim 1, wherein the removal of metal oxides iseffected by washing the metalcoated particles with a cleaning solution.

3. A method according to claim 2, wherein the cleaning solution is analkaline cleaning solution.

4. A method according to claim 2, wherein the cleaning solution is a 10to 30% by weight sodium carbonate solution.

5. A method according to claim 2 wherein the temperature of the cleaningsolution is adjacent its boiling point.

6. A method according to claim 2, and removing the cleaning solutionfrom the particles and drying the particles prior to deposition thereonof said protective layer.

7. A method according to claim 6, wherein said cleaning solution removaland drying are effected by contacting the particles with acetone.

8. A method according to claim 1, wherein the deposition of theprotective layer is effected by immersing the particles in a solution ofsaid resin in tetrahydrofuran.

1. A METHOD OF PRODUCING COATED ABRASIVE PARTICLES FOR USE IN THEMANFACTURE OF A RESIN-BONDED ABRASIVE TOOL IN WHICH THE COATINGCOMPRISES AN INNER METALLIC LAYER AND AN OUTER SOLID RESIN PROTECTIVELAYER, WHICH METHOD COMPRISES THE STEPS OF PROVIDING A BATCH OFPARTICLES COATED WITH A METAL SELECTED FROM THE GROUP CONSISTING OFNICKEL AND COPPER PRESENT IN AN AMOUNT FOR 20 TO 80% BY WEIGHT OF THECOATED PARTICLE, REMOVING METAL OXIDES FROM THE EXPOSED SURFACES OF THEPARTICLES, IMMEDIATELY THEREAFTER DEPOSITIING ON THE PARTICLES, BEFOREREOXIDATION OF THE EXPOSED SURFACE THEREOF OCCURS, A PROTECTIVE LAYER OFA SOLID RESIN SELECTED FROM THE GROUP CONSISTING OF PHENOLFORMALDEHYDE,EPOXY, POLYIMIDE, ALKYD, NONSATURATED POLYESTER AND POLYAMIDE IN ANAMOUNT FROM 1 TO 3% BY WEIGHT OF THE COATED PARTICLE, AND RECOVERING THEPARTICLES AS DISCRETE PARTICLES HAVING A METAL/PROTECTIVE LAYER COATING.2. A method according to claim 1, wherein the removal of metal oxides iseffected by washing the metal-coated particles with a cleaning solution.3. A method according to claim 2, wherein the cleaning solution is analkaline cleaning solution.
 4. A method according to claim 2, whereinthe cleaning solution is a 10 to 30% by weight sodium carbonatesolution.
 5. A method according to claim 2 wherein the temperature ofthe cleaning solution is adjacent its boiling point.
 6. A methodaccording to claim 2, and removing the cleaning solution from theparticles and drying the particles prior to deposition thereon of saidprotective layer.
 7. A method according to claim 6, wherein saidcleaning solution removal and drying are effected by contacting theparticles with acetone.
 8. A method according to claim 1, wherein thedeposition of the protective layer is effected by immersing theparticles in a solution of said resin in tetrahydrofuran.